In recent years, there has been an increasing market demand for a compact high-lift pump having a small liquid volume. A conventional centrifugal pump has a configuration in which impellers are disposed in multiple stages on a single shaft in order to achieve a high-lift property of the pump without having to increase an outer diameter of the pump. In this configuration, liquid is sequentially energized by the multi-stage impellers to be lifted to a high level.
A vertical multi-stage centrifugal pump includes impellers disposed vertically in multiple stages, where each of the impellers has discharge openings at its peripheral surface, and a suction opening at a bottom side thereof. FIGS. 5A to 5C illustrate an exemplary configuration of such an impeller. As shown in FIGS. 5A to 5C, the impeller has, along its circumference, a plurality of guide blades 127 for forming guide paths in tangential direction and a multiplicity of return blades 128 for forming return paths to collect liquid under pressure from the guide paths to a suction opening 137 of a next-stage impeller. The return blades 128 are radially provided between the impellers in multiple stages.
The return blades 128 are implemented by a multiplicity of thin ribs 132. The total volume of concave paths 133 formed between the ribs 132 is set to be larger than the total volume of non-path portions (ribs) of the whole region forming the paths (see, for example, Japanese Patent Laid-open Application No. 2003-184778). That is, defining the total volume of the flow paths to be V1 and the total volume of the non-path portions to be V2, V1 is larger than V2 (V1>V2).
However, the technique disclosed in Japanese Patent Laid-open Application No. 2003-184778 is mostly applied to a pump with a high liquid volume having a comparatively large suction port and discharge port. In case the technique is applied to a compact high-lift pump with a low liquid volume having a small suction port and discharge port, i.e., when it is applied to a pump having a low specific rate, the cross sectional areas of the paths of the guide blades and the return blades would become greater than the cross sectional area of the suction port to make the flow paths suddenly expanded, which would cause an increase of a liquid loss.